1/4 cup reduced-sodium soy sauce
2 tablespoons lemon juice
2 pounds grouper or red snapper fillets
1 medium onion, cut into 6 wedges and separated
3 teaspoons olive oil, divided
1 medium green pepper, sliced into thin rings
1 medium sweet red pepper, sliced into thin rings
1 medium sweet yellow pepper, sliced into thin rings
1 can (14-1/2 ounces) diced tomatoes, drained

In a small bowl, combine soy sauce and lemon juice. Pour 1/4 cup marinade in a resealable plastic bag; add fish. Seal bag and turn to coat; refrigerate for 20 minutes. Cover and refrigerate remaining marinade.
Meanwhile, in a large nonstick skillet, cook onion in 1 teaspoon oil for 1 minute. Stir in peppers; cook and stir for about 5 minutes longer. Stir in tomatoes; cook 1-2 minutes longer or until tomatoes are heated through and vegetables are crisp-tender. Remove from the heat; keep warm.
Drain and discard marinade from fish. In a large nonstick skillet coated with cooking spray, cook fish in remaining oil for about 8 minutes on each side or until fish flakes easily with a fork. Arrange two-thirds of pepper mixture on a serving platter; top with fish. Top with remaining pepper mixture and drizzle with reserved marinade. Serve immediately.